Recently, carbon nanotubes (hereinafter also referred to as “CNTs” for short) are widely researched and developed. Among the CNTs, single-walled CNTs are applicable to wide variety of uses because of, for example, their shape, electronic property, adsorption characteristics, mechanical characteristics and the like, so that their developments are strongly demanded.
Conventionally, an arc discharge method, a laser ablation method, and a CVD method are known as typical process for producing CNTs.
Among these processes, the arc discharge method is a method in which arc discharge is conducted between carbon rods under an argon or hydrogen atmosphere having a pressure little lower than atmospheric pressure to generate multi-walled CNTs in the deposits on the cathode. In this case, by performing the arc discharge using carbon rods containing a catalyst such as Ni/Y, single-walled CNTs can be generated on the inner surface of the vessel. The arc discharge method has an advantage that relatively good quality CNTs with few defects can be generated. On the other hand, however, it has problems in that i) amorphous carbon is simultaneously produced, ii) the cost is high, iii) it is not suited for synthesis in large scale, and so on.
The laser ablation method is a method in which CNTs are generated by irradiating a carbon containing a catalyst such as Ni/Co with a strong pulsed light such as YAG laser under an atmosphere at a high temperature between 900° C. to 1300° C. Although the method has advantages in that CNTs with a relatively high purity can be obtained, and the diameter of the tubes can be controlled by changing the conditions, the yield is small and employment of this method in an industrial scale is said to be difficult.
The CVD (Chemical Vapor Deposition) method is a method in which CNTs are generated by bringing a carbon compound serving as a carbon source into contact with catalyst metal particles at 500° C. to 1200° C. The method has variations in the type of the metal catalyst, the arrangement thereof, and in the type of the carbon compound, and both multi-walled CNTs and single-walled CNTs can be synthesized by changing the conditions. Further, by arranging the catalyst on a substrate, multi-walled CNTs aligned perpendicularly to the surface of the substrate can also be obtained.
Dai et al. disclosed a CVD method by which single-walled CNTs can be obtained using carbon monoxide as a raw material and iron carbonyl as a catalyst (Chemical Physics Letters, 260, 471-475, (1996)). This method is best suited for large scale synthesis because a raw material in the form of gas can be supplied, and the ratio of single-walled CNTs is said to be relatively high. However, this method has a disadvantage in that the synthesized single-walled CNTs generally have many defects. Further, to generate single-walled CNTs, a temperature of 900° C. or higher is required. Still further, since highly toxic carbon monoxide and iron carbonyl are used, the method is problematic from the viewpoint of safety. Although a number of other production processes for producing single-walled CNTs by CVD method have been proposed, it was found by actual trials that each of them has a problem in that the percentage of the single-walled CNTs in the CNTs is as small as 20% or lower.